1. Field of the Invention.
The present invention relates to the protective coating of polyolefin floor tiles. More particularly, the present invention relates to a system for applying a polyurethane coating to a polyolefin floor tile in a factory environment.
2. Related Art.
The term polyolefin refers to any of the largest genus of thermoplastics, which are polymers of simple olefins such as ethylene, propylene, butene, isoprene, and pentene, and copolymers thereof. Two of the more important members of this group are polyethylene and polypropylene, which together account for just under half of all thermoplastics produced in the United States.
In recent years polyolefins and other polymers have been used to create resilient flooring materials for use in athletic arenas such as basketball courts, tennis and racquetball courts, and so forth. An example of such a tile is shown in FIG. 1. These tiles vary in size, and may range from about 10xe2x80x3 to 12xe2x80x3 square by xe2x85x9cxe2x80x3 to xc2xdxe2x80x3 thick. Because they are typically configured as interlocking tiles having approximately the same size as traditional floor tiles, these flooring materials are easy to install. However, because of the polymer construction, the resulting floor surface is relatively susceptible to scratches and abrasion, and tends to lose its glossy appearance over time. This is a problem for athletic floors where an attractive, durable, and long lasting high gloss surface is desired.
To solve these problems, some sort of coating of the floor tiles is desirable. However, due to the chemical structure and simplicity of polyolefins and other polymers, their surfaces are generally resistant to any kind of permanent coating or decorating. Polyolefins, for example, are generally characterized by a nonpolar, nonporous, low-energy surface structure that does not easily bond to inks, lacquers, and other polymers without special oxidative pretreatment. The resistance of polyolefins to coating or decorating is especially problematic when the substance to be bonded is another polymer such as polyurethane. Polyurethane is well known and has many uses in biomedical and other applications. It""s suitability to these applications is due in large part to its very low reactivity: polyurethane is very inert, and resists reaction with body fluids and other organic and inorganic chemicals. Polyurethane would be an excellent coating for a polyolefin floor material because it can be made to have a scratch and abrasion resistant surface and a long lasting high gloss appearance.
In order to sufficiently bond a coating or decoration to a polyolefin or other polymer, the surface is ordinarily treated in some way, or a secondary adhesion-promoting layer is added to improve bonding. There are a number of common methods for doing this, including the use of heat and pressure, chemical treatment, electron bombardment, flame treatment, and plasma or corona treatment.
The application of pressure and temperature together can cause some coatings and decorations to bond to a polymer surface. An example of this method is hot stamping, which involves the use of a heated applicator and a special ink held by a foil backing. The ink is forced via heat and pressure to transfer to the new substrate. This method works quite well with some small sized parts and certain families of plastics. However, this technique is very sensitive to the size and shape of the objects to be treated. It generally only works well with small or flat surfaces that can be stamped or rolled. Large or convoluted shapes or surfaces that have complex geometric structure or texture are virtually untreatable using heat and pressure. Additionally, this technology requires specialized, stationary equipment. A preferred method of surface treatment will allow the treatment of large or oddly shaped parts and those with textured surfaces in addition to surfaces that may be stamped or rolled, and may be accomplished with small, simple equipment that may be easily moved.
Chemical treatment is of two kinds: chemical abrasion, and the application of a secondary xe2x80x98primerxe2x80x99 layer. Chemical abrasion involves the activation of the polymer surface with a solvent, and is typically used with polar materials. The solvent chemically xe2x80x98etchesxe2x80x99 the surface of the polymer, creating an abraded and/or chemically changed surface that is more conducive to bonding. Examples of chemical abrasion are the application of acetone or MEK to acrylic, styrene, PVC, and ABS. The use of a secondary primer layer involves the application of a material that, because of its own high level of chemical activity, will bond to both the polymer substrate and the coating or decoration. An example of such a primer would be a chlorinated compound held in a solvent emulsion.
There are a number of significant drawbacks to chemical treatment. First, if too strong of a chemical solvent is used, or exposure is too prolonged, the polymer will soften or dissolve. There are also significant dangers posed by human exposure to chemical solvents, and the introduction of these chemicals into the environment. A preferred method of increasing the surface energy of polymers will increase the surface energy enough to promote bonding, while avoiding the possibility of dissolution of the polymer itself, and prevent or limit human and environmental exposure to harmful chemicals.
Electron bombardment involves the direction of a beam or xe2x80x98cloudxe2x80x99 of electrons onto a plastic surface to interact with the surface. The free electrons in the cloud or beam act to knock existing electrons out of their orbital positions in the polymer molecules, creating locations on the surface where other chemicals may bond. The electron beam may also cross-link or cut some polymer chains, creating additional locations for chemical bonding. This process is carried out in a vacuum environment to minimize the effects of air molecules. The automotive industry commonly uses electron bombardment to activate bumper fascias and other large parts.
Electron bombardment is a very expensive method of polymer activation because it requires the placement of the object into a closed vacuum chamber. Moreover, with this method some areas of the surface will receive less treatment than others. A preferred method of polymer surface activation will treat all areas of a surface equally, will have a reasonable cost, and will not require the placement of the item into a vacuum chamber or other device of a fixed size, allowing the treatment of objects of variable size and shape in a normal human environment.
Flame treatment involves the brief application of a flame or heat to the polymer surface. This oxidizes a thin surface layer of the material, creating highly active surface molecules that will bond with inks, dyes and other coatings. However, flame or heat treatment alone does not always produce good results. Many polymers have difficulty withstanding the addition of heat without deforming or changing in clarity or physical structure. If excessive heat is applied, the material may soften or warp. Excess heat may also cause accelerated aging by the introduction of heat history to the material. Consequently, when the added heat is kept below a level which prevents these problems, the polymer frequently will not obtain sufficiently increased surface energy to adequately promote bonding. A preferred method of increasing the surface energy in polyolefins and other polymers will increase the surface energy enough to promote bonding, while limiting surface temperature increase to below a level which will deform or significantly damage the material.
Another method of treating a polymer surface to increase its surface energy that is superior in some ways to each of the above described methods is corona or plasma treatment. In the discipline of physics, the term xe2x80x9cplasmaxe2x80x9d describes a partially ionized gas composed of ions, electrons, and neutral species. This state of matter may be produced by either very high temperatures, such as exist in celestial bodies or nuclear explosions, or by strong electric arcs or electromagnetic fields. An electric arc plasma may be produced by a pair of electrodes spaced some suitable distance, facing each other. The electrodes are then given a high voltage charge (AC or DC), which causes electricity to arc across the gap between the electrodes. The distance between the electrodes primarily depends upon the voltage used. This high energy electric arc produces a plasma in the region immediately around the electric arc.
When a plastic surface is exposed to a high energy plasma produced by a high voltage electric arc, the plasma interacts with the surface molecules, increasing their energy through a variety of mechanisms, depending on the specific polymer involved. In some cases, surface hydrogen molecules are removed, leaving behind active bonding sites. Also, crosslinking or scission can occur in the surface molecules, as in electron bombardment. This will change the surface energy of the material, making it easier for a coating to adhere. Oxides may also form on the surface, as in flame treatment, which are easier to bond to than the actual base polymer. These are just a few of the possible chemical mechanisms which are caused by plasma treatment that increase surface energy. The great benefit of using electric arc plasmas is that they are relatively low temperature, and can be used without damage to the surface of polymers and other relatively delicate materials.
In spite of the variety of methods for surface treatment of polyolefins, each method presents drawbacks and/or limitations which reduce their effectiveness.
It has been recognized that it would be advantageous to develop a floor tile coating system that raises the energy level of the tile surface above what is possible with any one of the prior art treatment methods.
In one aspect, the invention advantageously provides a system for applying a polyurethane coating to a polyolefin floor tile. The system includes a conveyor for moving the floor tile past a number of treatment devices, including a heater, a plasma generator, a first applicator for applying liquid polyurethane, and a first ultraviolet light system. The heater and plasma generator increase the energy of the top surface of the floor tile, and the first applicator then applies a first coating of liquid polyurethane to the top surface of the floor tile while in the energized state. The ultraviolet light system exposes the first coating to ultraviolet light to at least partially cure it.
In accordance with a more detailed aspect of the present invention, the system includes a second applicator, for applying a second coat of liquid polyurethane atop the first coat, and a second ultraviolet light system, configured to at least partially cure the first coat and the second coat of liquid polyurethane, after they have been applied to the tile.
Additional features and advantages of the invention will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the invention.